A silicon-based optical communication device is a highly-promising technology that enables an optical functional element and an electronic circuit to be integrated on the upper side of a silicon platform, utilizing a CMOS technology. The silicon-based optical communication device functions at the optical fiber communication wavelengths of 1330 and 1500 nm for various systems such as an optical fiber for residential use and Local Area Network (LAN), for example.
Recently, passive devices such as silicon-based waveguides, optical couplers, and wavelength filters are really widely researched. Moreover, as an important technology of a means for managing an optical signal for such a communication system, active elements such as silicon-based optical modulators and silicon-based optical switches are offered and are getting a lot of attention. An optical switch or optical modulator that changes a refractive index by utilizing the thermo-optical effect of silicon operates at low speed and can only be used in devices with a modulation frequency of less than 1 Mb/sec. Therefore, in order to obtain the high modulation frequency required in more optical communication systems, electro-optical modulators utilizing the electro-optical effect are necessary.
Most currently-proposed electro-optical modulators are devices of changing an optical phase and optical intensity by changing a real part and imaginary part of a refractive index through a change in free carrier density in a silicon layer by utilizing the carrier plasma effect. In modulators utilizing the free carrier absorption, an output is directly modulated by a change in optical absorption propagating in Si.
An example of a structure of a general electro-optical modulator (e.g., see FIG. 1 of Patent Document 1 and FIG. 2 of Patent Document 2) is shown in a cross-sectional view of FIG. 18. This electro-optical modulator is a silicon-based electro-optical phase modulator utilizing the shape of a rib waveguide formed on the upper side of an SOI substrate. As shown in FIG. 18, in this electro-optical phase modulator, slab regions extending laterally are formed, as a p+-doped semiconductor silicon 4 and an n+-doped semiconductor silicon 5, on both end sides of the shape formed of an intrinsic semiconductor silicon region 1. The structure of the rib waveguide is formed utilizing an Si layer (buried oxide layer) 2 on the upper side a Silicon On Insulator (SOI) substrate 3. The structure shown in FIG. 18 is a structure of a PIN diode type modulator, and with this structure, the free carrier density in an intrinsic semiconductor region is changed by applying a forward bias and a reverse bias, and a refractive index is changed utilizing the carrier plasma effect. In this example, the intrinsic semiconductor silicon layer 1 is formed so as to include a p-type region 4 doped so as to have a high doping concentration in a region of being in contact with a first electrode contact layer 6. In FIG. 18, the intrinsic semiconductor silicon layer 1 further includes the n-type region 5 doped so as to have a high doping concentration and a second electrode contact layer 6 connected thereto. In the structure of the above-described PIN diode, the regions 4 and 5 can be doped so as to each have the carrier density of about 1020/cm3. Moreover, in the structure of the above-described PIN diode, the p-type region 4 and the n-type region 5 are arranged on the both end sides of the rib 1 at a distance, and the rib 1 is the intrinsic semiconductor. The upper surface of the intrinsic semiconductor silicon 1 is covered with an oxide cladding 7.
Patent Document 1 proposes a silicon-based electro-optical modulator including a main body region having a second conductivity type and a gate region having a first conductivity type that is different from the second conductivity type of the main body region, being stacked on the main body region so as to partially overlap therewith, wherein a relatively thin dielectric layer is formed on the interface of the stack. The silicon-based electro-optical modulator with the SIS (silicon-insulator-silicon) type structure of Patent Document 1 is illustrated in the cross-sectional view of FIG. 19. As shown in FIG. 19, this silicon-based electro-optical modulator is formed on the upper side of an SOI platform composed of a supporting substrate 3 and a buried oxide layer 2 formed thereon. The main body region 8 is formed as a relatively thin p-doped silicon surface layer configuring an SOI substrate. The gate region 9 is formed as a relatively thin n-doped polycrystalline silicon layer stacked on the upper side of the SOI substrate (main body region) 8 via an insulating layer. The gate region and the main body region are doped, and the doped regions are defined so that the change in the carrier density is controlled by an external signal voltage. One end portion of the main body region 8 is formed as a p+-doped semiconductor silicon 4, and an electrode contact layer 6 is formed thereon. One end portion of the gate region 9 is formed as an n+-doped polycrystalline silicon 10, and another electrode contact layer 6 is formed thereon. The upper surfaces of the main body region 8 and the gate region 9, and a spaces between the main body region 8 and the SOI platform and between the gate region 9 and the same are filled with an oxide cladding 7. In this silicon-based electro-optical modulator, an optical phase modulation is carried out by accumulation, removal, or inversion of free carriers in the both end sides of the dielectric layer. Ideally, an electric field of an optical signal agrees with a region in which the carrier density is dynamically subjected to external control.